A recent article in the Oct. 1990 issue of Scientific American by Barth et al. describes boron neutron capture therapy for cancer. The use of boron compounds in the treatment of human cancer is based on the unique affinity of nonradioactive .sup.10 B nuclei for thermal (low energy) neutrons. The major nuclear reaction occurring on slow neutron absorption by .sup.10 B gives an alpha particle of 1.47 MeV, a lithium atom of 0.84 MeV and a gamma ray of 0.48 MeV. One of the most attractive features of antineoplastic therapy involving such reactions is that reactants of very low energy (less than 1-2 KeV) are converted to cytotoxic products of approximately 2.8 MeV directly within the cancerous cell. Since the nuclear fragments produced by this fission reaction travel only about 10-15 micrometers or one cell diameter, destructive radiation predominates only in the immediate vicinity of cells containing significant .sup.10 B concentrations.
None of the normal elements comprising human tissue possess significant thermal neutron cross sections compared to .sup.10 B, although their high normal concentrations mean that some 10-15% of the neutron dose is absorbed in side reactions. These are principally the H(n,.gamma.)D and .sup.14 N(n,P).sup.14 C reactions as well as whatever fast neutron and gamma contaminants are present in the incident beam. Experience has shown that these contaminating gamma rays and fast neutrons can be removed, and appropriately filtered beams may be designed to give good neutron penetration to any desired depth to reach deep-seated tumor sites. Boron-10 has a thermal neutron cross section of approximately 4000 barns and a natural abundance of nearly 20%. Moreover, current technology permits relatively facile and inexpensive enrichment of boron compounds up to about 95% including the borane starting materials noted in this application. The thermal neutrons themselves are of subionizing energy and no significant effect from thermal neutron irradiation in human tissue has been noted in the literature.
The only requirements for effective clinical use of boron neutron capture theory (BNCT) in treating human cancer are: (1) a significant concentration of .sup.10 B in the neoplasm, historically estimated to be 10-20 .mu.g of .sup.10 B per gram tissue (but potentially as low as about 1 .mu.g), and (2) a ratio of tumor localized .sup.10 B to plasma .sup.10 B sufficient to avoid adverse effects on normal tissue in the immediate vicinity of the tumor, especially the vascular endothelium.
In the 50 years or so since the use of thermal neutron fissioning of .sup.10 B as a therapeutic modality was proposed, chemists have been faced with the challenge of developing boronated molecules which would specifically and selectively concentrate in neoplastic cells. Until fairly recently, this has proven to be a tantalizing but unattainable goal. None of the many boron compounds synthesized and evaluated during the period from 1950 through the early 1980's has been found to be truly suited for BNCT.
A resurgence of interest in selective targeting of boron compounds had occurred in the early 1970's. Several groups have since bound boron to antibodies in various forms, but generally found that the resultant conjugates either precipitated, could not be loaded with sufficient boron, or could not be purified Hawthorne, et al., J. Med. Chem. 15:449 (1972); Wong, et al., J. Med. Chem. 17:785 (1974); and, Sneath, et al., J. Med. Chem. 17:796 (1974).
The failure of efforts up to about 1980 resulted primarily from lack of tumor specificity and excessive blood boron concentration at the time of irradiation. Since 1980 a second resurgence of interest in the area occurred. This work has generally taken the form of attaching boron in some form to biological carriers specifically targeted to cancerous cells. Na.sub.2 B.sub.12 H.sub.11 SH (BSH) is currently being utilized by Dr. Hiroshi Hatanaka, a Japanese neurosurgeon, in the treatment of upwards of 100 patients. His clinical studies of BNCT treatment of grade III-IV cerebral gliomas with this compound have shown five year survival rates of 30%, a figure far exceeding that obtained with conventional techniques. Hatanaka, Boron neutron Capture Therapy for Tumors, Nishimura (1986). BSH is currently being considered for clinical trials in the United States and Europe despite the fact that it does not show particularly strong preference for tumor cells of any type and almost certainly does not remain within or enter tumor cells for any significant time. Very recently another Japanese group, led by dermatologist Dr. Yutaka Mishima, has announced the successful treatment of eight cases of human malignant melanoma using p-boronophenylalanine (BPA) as the boron carrier. Mishima, et al., Lancet 2:388 (1989). This compound is tumor cell-targeted by virtue of the excessive uptake of aromatic amino acids by melanotic melanoma cells where it is believed to serve as a false precursor for melanin biosynthesis, though the available evidence suggests it is not a tyrosinase substrate. BPA appears to be taken up in therapeutic amounts by the KHJJ murine mammary carcinoma, the GS-9L rat glioma, and the human U-87 MG glioma xenograft in nude mice. This compound delivers only a single boron atom per unit carrier, must be given in massive doses, and crosses the blood-brain barrier to a significant extent making it inappropriate for BNCT of glioma.
Other American groups have reported the synthesis of boronated pyrimidines and nucleosides, monoclonal antibodies, chlorpromazines, and amino acids.
U.S. Pat. No. 4,959,356, issued Sep. 25, 1990, inventors Miura and Gabel describe boronated porphyrin compounds for use in BNCT, such as compounds of the formulas ##STR3## wherein R is hydrogen or lower alkyl having 1 to 6 carbon atoms and n is 0 to 10 and metal salts thereof. Similar is an article by Miura et al. Tetrahedron Letters, Vol. 31, No. 16, pp. 2247-2250 (1990). However, the boronated porphyrins described by Miura have vinyl carborane moieties that she reports as not being water soluble. Thus she must open the borane cages. But by opening those borane cages, one encounters significantly more toxicity for the compounds. Moreover, the resultant open-cage compounds are still not sufficiently water-soluble to enable administration without the use of adjuvant substances (e.g., polyethylene glycol). Also the compounds which Miura et al. describe are (at most) 19% boron by weight in the physiologically useful (K.sup.+ -salt) form. This is a disadvantage since limiting human doses may well be determined by the amount of porphyrin unit doses which may be tolerated.
U.S. Pat. No. 4,963,655, issued Oct. 16, 1990, inventors Kinder et al., describes compounds of the formula: ##STR4## or physiologically acceptable salts thereof.
In addition to neutron capture therapy (NCT) generally and boron neutron capture therapy (BNCT) more specifically, additional uses of porphyrins in cancer therapies are those therapeutic strategies generally referred to as photodynamic therapy (PDT). A review article by Delaney and Glatstein in Comprehensive Therapy, pp. 43-55 (May 1988) describes this therapeutic strategy where a light-activated photosynthesizer can interact with ground state molecular oxygen to yield reactive oxygen species (via singlet oxygen). Since porphyrins of many structural types localize in a wide variety of malignant tumors, this localization has formed the basis for treatment of at least 3000 patients in the United States alone (twice that worldwide) over the past several years through PDT. Complete response (disappearance of tumor or biopsy proven) has occurred in a high percentage of patients in relatively advanced stages of skin, bladder, and lung cancers, and cancers of the reproductive system through photodynamic therapy.
Hematoporphyrin derivative (HPD) and Photofrin II composition, the PDT agents used to date in clinical trials, are complex mixtures of porphyrins. Aggregated hematoporphyrin dimer and trimers bound together with ester linkages appear to be the most likely structures o the tumor-localizing fractions of HPD when isolated by LH-20 gel filtration using organic solvents. Other evidence suggests that an ether linked oligomer fraction contributes significantly to the localizing fraction. Whatever the covalent linkage, these polymers have a very strong tendency to aggregate in aqueous media through non-covalent forces. This form is neither fluorescent nor an efficient singlet oxygen generator.
A variety of new drugs structurally related to porphyrins are being investigated for photodynamic therapy. Among these are phthalocyanines, chlorins, and purpurins. The phthalocyanines (PC) have received the greatest share of attention and have been found to be selectively retained by tumors, are resistant to chemical and photochemical degradation, are non-toxic, and are relatively easy to synthesize. PC's containing a fissionable or neutron-activatable nuclide were even proposed for treatment of brain tumors 25 years ago. Both in vivo and in vitro studies with sulfonated PC's indicate that tumor cell localization and photoinduced tumoricidal activity for several water-soluble PC's are similar to HPD. Particularly noteworthy is the lack of toxicity of various PC's in various species. Other compounds which have recently received attention as photosensitizers are chlorins and purpurins, and benzoporphyrins (BPD).
However, given the clear superiority of NCT to PDT in the treatment of many types of deep-seated tumors, it would appear that boronated porphyrins offer one of the best hopes for clinical use of NCT. Moreover, the lessons learned in NCT localization should be applicable to PDT, and vice versa, and in the case of boronated porphyrins might permit simultaneous use of both NCT and PDT.
A recent report by a blue-ribbon DOE panel concluded "There is a need for boron compounds which possess greater specificity for tumor, longer persistence and low systemic toxicity. Compound development of those structures which possess both specificity and persistence for tumor cells under in vivo conditions, while attaining low values in blood and continuous normal structure, is crucial. Rational syntheses of boron compounds and a well designed biological evaluation are important not only to determine concentration and persistence per se but to provide feed back to the synthetic chemist in the creation of new, more useful compounds." Report of the Health and Environmental Research Advisory Committee, (p. 20) S. Wolff, chair; Office of Energy Research, Department of Energy, 1990.